In a seismic survey a source is actuated to generate seismic energy, and the resultant seismic wavefield is sampled by an array of seismic receivers spaced from the seismic source. Each receiver acquires seismic data, which are normally in the form of a record or “trace” representing the value of some characteristic of the seismic wavefield against time. The acquired seismic data are a representation of the seismic wavefield at the receiver location. Information about the earth's sub-surface can be obtained from the acquired seismic data.
One well-known type of seismic receiver is the seismic geophone. A geophone contains one or more sensors mounted in a casing. A geophone may be single component geophone, which contains one sensor that records the component of the seismic wavefield parallel to a pre-determined direction. Information about the vertical component of the seismic wavefield, for example, may be obtained using a single component geophone oriented such that the sensing direction of the geophone is substantially vertical. Alternatively, a geophone may be a three-component geophone which contains three sensors oriented so as to record the components of the seismic wavefield in three orthogonal directions (normally denoted as the geophone's x-, y- and z-axes).
A geophone may be deployed simply by placing the geophone casing on the earth's surface. This may be done in land-based surveys or in marine surveys where geophones are deployed on the sea-floor. Alternatively, geophones in which the casing is provided with a spike are known, and these are deployed by driving the spike into the earth's surface. Spiked geophones are generally limited to use in land-based surveys.
In order accurately to measure the seismic wavefield, a geophone must be well coupled to the Earth's surface. The output from a geophone is the seismic wavefield convolved by the transfer function between the ground and the geophone. The transfer function represents what is commonly called the “coupling” of the geophone. If the coupling between the geophone and the ground is perfect, the transfer function is equal to one for all frequencies and the output signal is a good representation of the seismic wavefield. If the transfer function is not equal to unity for all frequencies of interest, however, then the output signal from the geophone is a distorted representation of the seismic wavefield. Where a multi-component geophone is used, there will be a separate transfer function for each component of the seismic wavefield that is sensed by the geophone or, if mutual effects are not negligible, a multi-channel transfer function.
Acknowledgement of the Prior Art
The study of geophone transfer function has formed the subject of several publications. It has been recognised for some time that the motion of a geophone case resting on the Earth's surface is not the same as the motion of the earth that would occur in the absence of the geophone case. The coupling between a geophone and the earth was investigated by Washburn, H. and Wiley, H. in “The effect of the placement of a seismometer on its response characteristics”, Geophysics, Soc. of Expl. Geophys., 06, 116-131 (1941) and by Wolf, A. in “The equation of motion of a geophone on the surface of an elastic earth”, Geophysics, Soc. of Expl. Geophys., 09, 29-35 (1944). However, these classical theoretical works are based upon a model of a geophone with a flat base resting on the Earth's surface, and this would be expected to have different coupling to the ground from the nowadays commonly used spiked geophone.
Drijkoningen, G. G. has reported, in “The usefulness of geophone ground-coupling experiments to seismic data”, Geophysics, Soc. of Expl. Geophys., 65, 1780-1787 (2000), field experiments on geophone coupling and has proposed a two-state system to explain the phenomena that occur with planted spike geophones: spike coupling and gravitational coupling. These two states correspond respectively to a firmly planted geophone and to a geophone resting on its base. In the first state, the coupling resonance frequency is (at least for vertical component geophones) beyond the seismic bandwidth, but in the second case (of gravitational coupling) the coupling resonance frequency may, depending on the soil firmness, be within the seismic bandwidth and therefore affect the amplitude and phase of the signal recorded by the geophone. In this case the signal recorded by the geophone is no longer proportional to the ground particle velocity. (The term “coupling resonance frequency” denotes the frequency at which a resonance, arising as a result of the geophone coupling, occurs in the transfer function). Gravitational coupling is the mechanism upon which most of the seabed seismic acquisitions are based, but it is undesired in a land-based seismic survey where spike coupling is expected to be the dominant mechanism.
Krohn, C. E. has reported, in “Geophone ground coupling”, Geophysics, Soc. of Expl. Geophys., 49, 722-731, (1984), laboratory and controlled small scale field experiments leading to the conclusion that good planting of modern vertical component sensing geophones having spikes up to 5 inches long is acceptable for conventional seismic surveys using frequencies up to 100 Hz and in which the particle velocities at the geophone are less than 10−2 cm/s. However, in commercial size acquisitions, the quality of geophone planting cannot be guaranteed throughout the surveyed area, and it is also possible for a well-planted geophone to be disturbed after planting. Moreover, in sand-covered areas geophones without spikes are often used, buried a few centimetres below the surface. A simple method of detecting bad coupling conditions is highly desirable, as this would allow data acquired by a badly-coupled geophone to be corrected to compensate for the bad coupling.
When a multi-component geophone is deployed, coupling between the geophone and one component of the seismic wavefield may be better than the coupling between the geophone and another component of the seismic wavefield. U.S. Pat. No. 5,724,307 proposes a method for determining a filter that corrects for imperfect coupling of a geophone to the cross-line component of the seismic wavefield on the assumption that the geophone is perfectly coupled to the in-line component of the seismic wavefield.
However, this method will give good results only if the geophone is well coupled to the in-line component of the seismic wavefield.
U.S. Pat. No. 5,724,306 proposes a method for determining a filter that corrects for imperfect coupling of a geophone to a component of the seismic wavefield. This method is applicable to a seismic receiver that contains a geophone and a hydrophone, and is based on the assumption that the hydrophone is perfectly coupled. This method cannot, however, be applied to a receiver that does not contain a hydrophone and so cannot be used in a land-based seismic survey.